Hermetically sealed containers are important in many applications. For example, crystal resonators are widely used as a secondary frequency standard or as a frequency source in many different electronic applications. It is of prime importance for the proper operation of electronic devices including such a resonator that the resonator have stable frequency characteristics over a period of time.
As is noted in Hafner et al. U.S. Pat. No. 3,931,388, several factors can cause changes in the frequency characteristics of crystal resonators, e.g., changes in mass of the resonator, the pressure inside the resonator enclosure, stress on the resonator, electrode to quartz interfaces, and the quartz's crystal structure. The pressure changes inside the resonator enclosure can be caused by, inter alia, outgassing and leaks due to flaws in the packaging seals. Thus, one method of increasing the stability of such frequency characteristics is to enclose the crystal resonator in a hermetically sealed container to reduce such pressure changes.
The Hafner et al. '388 patent discloses a crystal resonator package which is said to avoid some of the above-noted problems. Specifically, an apparatus is disclosed for housing a crystal resonator comprising an insulator "frame-like" housing having an opening capable of surrounding a major portion of the crystal, wherein the frame member includes a metallic conducting material embedded in a sidewall of the frame and directed from the opening to an outside wall of the frame, and wherein at least one support member is secured to the frame and within the opening. The support member is electrically coupled to the metallic conducting material and is capable of supporting the crystal in a desired vibratory mode. The patent also mentions means coupled to the frame for hermetically sealing the crystal within the opening. As the hermetically sealing means, the Hafner et al. '388 patent suggests frame members 12 having top and bottom surfaces which are metallized and a lid member 40 corresponding to the top surface of the frame member 12 having underneath surfaces which are also metallized. Hafner et al. also suggest that a wire O-ring can be used between the metallized surfaces to compensate for small surface irregularities and contaminants. The metallized surfaces are used to form the hermetic seal. For example, in columns 5 and 6 of the Hafner et al. patent, they suggest that such metallized surfaces can be sealed by a number of different methods, including cold welding, thermocompression bonding, electron beam welding, laser welding, parallel seam and other resistence welding, ultrasonic bonding and reflow soldering.
A similar container for crystal resonators is also disclosed in Snow et al. U.S. Pat. No. 3,951,327. The Snow et al. patent discloses a ceramic to metal seal which is said to provide a hermetic seal. The metal to ceramic seal is said to be formed by heating a wire-like metal gasket and chemically cleaned ceramic member, while simultaneously deforming from about 50 to 95% the metal gasket against the ceramic member at a temperature of about 30 to 75% of the melting temperature of the metal gasket.
A method of packaging crystal resonators is also disclosed in Hafner et al. U.S. Pat. No. 3,914,836. This Hafner et al. patent discloses a method for processing crystal resonators in which certain process steps are performed in a vacuum system without venting between stages, including processing steps of cleaning the resonator parts to remove contaminants from the surface, baking the resonator parts at temperatures up to 450.degree. C. to further remove adsorbed and absorbed contaminants, plating electrodes onto the crystal resonator, and sealing the resonator parts. As the sealing means, Hafner et al. in column 2 suggest cold welding of similar metal surfaces under near zero pressures or by compression welding in situations where the surfaces are contaminated. The former sealing method is said to rely on the adhesion between atomically cleaned surfaces. The Hafner et al. patent also mentions that one or more small diameter O-rings of a suitable material plus pressure and heat (if necessary) can be used to compensate for small surface irregularities and contaminants. As an alternative, Hafner et al. suggest that O-rings can be replaced with one or more ridges on one of the sealing surfaces.
Containers for crystal resonators and methods for making such containers are also disclosed in Technical Report ECOM 4134, "Packaging Crystal Resonators" by J. Vig and E. Hafner, July 1973.
In summary, any improvement in the pressure characteristics of a container enclosing a crystal resonator is important in many applications since stabilizing the pressure will help to create a more constant frequency. Thus, while good hermetic seals can be obtained by the containers and methods of the prior art, any improvement in such hermetic seals would be highly desirable.